The Great War ended 100 years ago this month. How does it still shape our world now? Here's everything you need to know:

What caused the war?In 1914, the great powers of Europe were enmeshed in a tangled web of alliances that had formed over decades of colonial empires jockeying for dominance. The assassination of the Austro-Hungarian Archduke Franz Ferdinand by a 19-year-old Bosnian Serb activist started a chain reaction that plunged these nations into a cataclysmic struggle. Austria-Hungary, which had been looking for an opportunity to project strength in the Balkans, declared war on Serbia, accusing its government of orchestrating the attack. Russia then mobilized to defend its ally Serbia. This led Germany, which was allied with Austria-Hungary, to declare war on Russia and its ally France, and to invade France's neighbor, neutral Belgium. Britain, which had promised to protect Belgian neutrality, then declared war on Germany, which it had been battling for naval supremacy. In the four years that followed, some 9.7 million soldiers and 10 million civilians died in a conflict so ghastly that many survivors returned with "shell shock," haunted by what they had witnessed.

Why did so many die?Technology. The introduction of machine guns, barbed wire, and highly accurate artillery made advancing over open ground tantamount to a suicide run. Nevertheless, military leaders still clung to 19th-century tactics for much of the war, ordering massed infantry assaults meant to overrun enemy positions. But when soldiers left their trenches and went "over the top," they were mowed down by the thousands. The industrialization of war produced death on an unprecedented scale; in France, for example, 13.3 percent of the male population between the ages of 15 and 49 died in the war. The fighting provided a grim preview of even greater horrors to come, with the first widespread use of military aircraft, bombing of civilians, chemical weapons, and armored tanks. World War I was "the first calamity of the 20th century," wrote historian Fritz Stern. "The calamity from which all other calamities sprang."

What followed the war?The war's end set the stage for a new series of global conflicts, some of which are still raging today. In Russia, war fatigue led directly to the collapse of the centuries-old Romanov dynasty, the 1917 Russian Revolution, and the establishment of Communist rule. The polyglot empire of defeated Austria-Hungary was dissolved into a collection of independent states based on ethnic identity, including former Yugoslavia, that are riddled with nationalist and sectarian tensions to this day. The harsh terms imposed by the victors in the Treaty of Versailles helped lead to a surge of nationalism in Germany, and ultimately to the rise of Adolf Hitler and his Nazi Party. Some historians see World War I as the beginning of a continuous struggle for Europe that didn't really end until the reunification of Germany in 1989.

What about the rest of the world?After World War I, the allies stripped Germany of its colonies in Asia and Africa. But instead of being given independence, these long-oppressed lands were absorbed into the victors' colonial empires. Colonized peoples resented being denied the right to national "self-determination" extended to newly created or liberated European countries like Poland, fueling independence movements in India and several African nations. World War I also redrew the map of the Middle East. The British and the French carved up the remnants of the Ottoman Empire, which had entered the war on Germany's side. Under the Sykes-Picot agreement, France claimed Lebanon and Syria for its sphere of influence, while Britain took control of what became Iraq and Jordan, as well as the Gulf States. The new borders were arbitrarily drawn, with no regard for long-standing religious and tribal identities. Iraq, for example, was created by lumping three former Ottoman provinces together, dominated respectively by Shias, Sunnis, and Kurds. When ISIS swept across Syria and Iraq, its leader, Abu Bakr al-Baghdadi, declared his intention to erase the old colonial borders. "This blessed advance will not stop until we hit the last nail in the coffin of the Sykes-Picot conspiracy," he said.

What was the impact on the U.S.?America reluctantly entered the war on the side of the allies in 1917, but its late intervention powered the exhausted and nearly bankrupt allies to victory. The war made the U.S. the world's leading creditor, shifting the seat of global finance from London to New York City. Untouched at home by the ravages of war while Europe was devastated, America saw its economy boom, surpassing the British Empire's to become the largest in the world. President Woodrow Wilson hoped to shape a postwar order with the League of Nations, which was designed to prevent future wars. But the Senate rejected joining the League, with opponents calling it incompatible with American sovereignty. Nevertheless, Wilson's declaration that "the world must be made safe for democracy" set a precedent that has endured. "That has been the foundation of almost all American foreign policy for the last 100 years," said historian A. Scott Berg. "Whether you agree with it or not."

The meaning of Armistice DayThe First World War effectively ended on Nov. 11, 1918. For the victors, Nov. 11 was immediately recognized as a day of celebration and thanksgiving. In the U.S., Armistice Day was celebrated until 1954. In the aftermath of World War II and the Korean War, Congress changed the holiday to Veterans Day in order to honor all American veterans. But not everyone agreed with the name change. "Armistice Day was sacred," World War II veteran Kurt Vonnegut wrote in his 1973 novel Breakfast of Champions. "Veterans Day is not." Veterans for Peace, an anti-war group, holds regular "Reclaim Armistice Day" events on Nov. 11, arguing that the day was originally meant to celebrate peace, not militarism. "Armistice Day was a hallowed anniversary because it was supposed to protect future life from future wars," says Rory Fanning, a veteran of the Afghanistan war who became a conscientious objector. "Veterans Day, instead, celebrates 'heroes' and encourages others to dream of playing the hero themselves, covering themselves in valor."